1. Field of the Invention
The present invention relates to an exhaust gas purifier for use in an internal combustion engine having, in an exhaust gas passage thereof, a catalyst device (NOx catalyst) which occludes NOx contained in the exhaust gas at a lean air-fuel ratio; i.e., in an oxygen-excessive atmosphere, and which releases occluded NOx at a stoichiometric air-fuel ratio or a rich air-fuel ratio; i.e., in a low-oxygen-concentration atmosphere.
2. Description of the Related Art
Recently, in order to improve fuel economy, a lean-burn engine enabling combustion at a lean air-fuel ratio has been put into practical use. The lean-burn engine has a problem in that a conventional three-way catalytic converter cannot sufficiently purify NOx in an exhaust gas during a lean combustion due to its purification characteristics. Thus, recently there has been employed, for example, a catalyst device (NOx catalyst) for purifying NOx in the exhaust gas during the lean combustion through occlusion of NOx contained in the exhaust gas.
The NOx catalyst is known to purify NOx contained in exhaust gas in an oxygen-excessive atmosphere (at a lean air-fuel ratio) through occlusion of NOx onto a catalyst and to release the occluded NOx when an oxygen concentration lowers (at a stoichiometric air-fuel ratio or a rich air-fuel ratio). Specifically, in the oxygen-excessive atmosphere, the NOx catalyst produces a nitrate from NOx contained in exhaust to thereby occlude NOx. By contrast, in a low-oxygen-concentration atmosphere, the NOx catalyst causes the nitrate occluded on the NOx catalyst and CO contained in the exhaust gas to react with each other, thereby generating a carbonate and releasing NOx.
In the oxygen-excessive atmosphere during a lean operation, the NOx catalyst occludes NOx thereon. However, when the NOx catalyst becomes saturated with occluded NOx after continuation of the lean operation, most NOx contained in the exhaust gas is emitted into the atmosphere. Thus, before the NOx catalyst becomes saturated with NOx, the air-fuel ratio is switched to a stoichiometric air-fuel ratio or a rich air-fuel ratio to lower the oxygen concentration of the exhaust gas, whereby NOx is released and reduced to thereby restore the NOx occlusion capability of the NOx catalyst. According to a technique disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 7-166913, when the air-fuel ratio of the engine is switched to a stoichiometric air-fuel ratio or a rich air-fuel ratio in order to restore the NOx occlusion capability of the NOx catalyst, the air-fuel ratio is gradually changed to a stoichiometric air-fuel ratio or a rich air-fuel ratio to thereby release and reduce NOx while suppressing a torque shock acting on the engine.
When NOx is released and reduced through switching the air-fuel ratio of the engine to the stoichiometric air-fuel ratio or the rich air-fuel ratio (CO is generated and supplied into the exhaust gas; i.e., to the NOx catalyst) in order to restore the NOx occlusion capability of the NOx catalyst, a portion of the supplied CO is consumed for releasing the occluded NOx, and a residual CO is consumed for reducing released NOx. When a ratio, at which NOx is reduced by means of reducers, such as the residual CO and HC, coincides with that at which NOx is released, release of NOx and CO into the atmosphere can be suppressed.
However, the technique disclosed in the above publication encounters a difficulty in establishing coincidence between a ratio at which NOx is reduced and that at which NOx is released. This is because the NOx occlusion capability restoration performance of the NOx catalyst; i.e., the releasability of the NOx catalyst with respect to the occluded NOx (NOx-releasing rate), depends on the form and amount of a catalytic component carried by the NOx catalyst.
In the case of employment of a NOx catalyst having improved NOx occlusion capability restoration performance, the NOx-releasing rate, or the rate at which NOx is released from the NOx catalyst, is also improved. As a result, the amount of NOx present in the exhaust gas and to be reduced by means of reducers tends to become smaller than that of NOx to be released (NOx to be reduced<NOx to be released). Thus, residual NOx which remains in the exhaust gas without being reduced is emitted into the atmosphere. By contrast, in the case of employment of an NOx catalyst having a limited NOx occlusion capability restoration performance, the amount of NOx to be reduced tends to become greater than that of NOx to be released (NOx to be reduced>NOx to be released). Thus, reducers (CO, etc.) remain in the exhaust gas and are released into the atmosphere.
Generally, as the air-fuel ratio of the engine approaches the rich side (as the amount of CO increases), the NOx-releasing rate increases. Thus, when the air-fuel ratio is shifted toward the stoichiometric air-fuel ratio or the rich air-fuel ratio as described in the above publication, the NOx-releasing rate begins to increase at a near stoichiometric air-fuel ratio, at which the amount of CO begins to increase; thus, the amount of NOx to be released from the NOx catalyst increases. However, the amount of reducers (residual CO, HC, etc. which have not contributed to release of NOx) is not sufficient for reducing the increased amount of released NOx. As a result, the released NOx, remaining in the exhaust gas, is released into the atmosphere without being reduced.
A conceivable solution to this problem is to increase the amount of reducers by enriching the air-fuel ratio of the engine. In this case, since the amount of CO serving as a reducer also increases, the amount of NOx to be released increases. Therefore, in actuality, the above-mentioned “NOx to be reduced<NOx to be released” relation remains unchanged. As a result, the residual NOx, which remains in the exhaust gas without being reduced is released into the atmosphere, constituting failure to suppress emission of NOx.
Accordingly, the technique disclosed in the above publication encounters a difficulty in establishing substantial coincidence between the amount of NOx to be reduced and that of NOx to be released and thus involves a problem in that exhaust gas characteristics are impaired during release of NOx from the catalyst and reduction of released NOx.
The present invention has been accomplished in view of the foregoing, and an object of the present invention is to provide an exhaust purifier for use in an internal combustion engine capable of reliably reducing NOx released from a catalyst device (NOx catalyst).